One example of variable-powered binoculars that are able to vary their magnifying power continuously is a pair of binoculars with ocular zoom lens systems. The ocular lens system of this type of binoculars is provided with at least a first lens placed closer to the eye and a second lens placed near the concave adjusting lens. Sometimes, a third lens is further placed between the first and second lenses. With the binoculars having ocular zoom lens systems, the user can continuously vary their magnifying power by moving the second lens closer to or away from the first lens. This type of binoculars can vary the magnifying power largely through a small movement of the second lens along the optical axis, which leads to an advantageous miniaturization of the binoculars. Due to this advantage, almost all of the existing variable-powered binoculars employ the ocular zoom lens system.
FIG. 1 is a schematic illustration of the optical system of the conventional variable-powered binoculars with an ocular zoom lens system. FIG. 1(A) shows the optical arrangement when the system has a low magnifying power of 7, while FIG. 1(B) the arrangement when the system has a high magnifying power of 15. In FIGS. 1(A) and 1(B), reference numeral 1 depicts an objective lens, reference numeral 2 an ocular lens, reference numeral 3 a prism system, reference numeral 4 a concave adjusting lens, reference numeral 5 a field lens, reference numeral 6 an objective image forming plane, and reference numeral 7 an apparent image forming plane. In the optical system of FIG. 1, the objective lens 1, the ocular lens 2 and the prism system 3 are fixed, while the concave adjusting lens 4 and the field lens 5 are movable along the optical axis of the optical system in FIG. 1. When the magnification is small as shown in FIG. 1(A), the concave adjusting lens 4 is located at position a, and the field lens 5 at position c. To change the magnification from a low one shown in FIG. 1(A) to a high one shown in FIG. 1(B), the concave adjusting lens 4 is moved to position b in FIG. 1(B) and the field lens 5 to position d in the same figure. These two lenses are moved by turning a zoom lever or a zoom ring (not shown in the figures) which the variable-powered binoculars have. When the zoom lever or zoom ring is turned, the concave adjusting lens 4 and the field lens 5, which are interlocked, shift together along the optical axis of the optical system. When the user of the variable-powered binoculars would like to see a magnified image of an object, s/he needs to turn the zoom lever or zoom ring to a position where the lens system gives him/her the desired magnification.
However, since the conventional variable-powered binoculars have a mechanism in which the magnification is changed by moving two lenses, the concave adjusting lens and the field lens, they have difficulties in providing a large visual field and keeping a high resolution. It is because the quality of an image tends to change, which is caused by change in stigmatism when the magnifying power was changed from a low one to a high one. Besides, because of the difference between the eye relief when the arrangement is set for a high magnification and that when the arrangement is for a low magnification, the user sometimes cannot see some objects very well. Moreover, since the concave adjusting lens and the field lens have to be interlocked in the conventional variable-powered binoculars, the mechanical structure to interlock them is complicated, and therefore the production or assembly of the binoculars is not always easy.